paperback vs paperback book

paperback and paperback book both are nouns.

Word	Noun	Adjective	Verb	Adverb
paperback	Yes	Yes	No	No
paperback book	Yes	No	No	No

As nouns, paperback and paperback book are synonyms defined as:

Other synonyms of paperback include paper-back book, soft-cover, soft-cover book, softback, softback book.

paperback (noun)	paperback book (noun)
a book with paper covers	a book with paper covers

paperback (adjective)	paperback book (adjective)
(of books) having a flexible binding